Ultraviolet light emitting elements have been widely attracting attention as next-generation light sources used for illumination, sterilization, photolithography, laser machines, medical equipment, light sources for phosphors, spectral distribution analysis, ultraviolet curing, and so on. Such an ultraviolet light emitting element includes a group III nitride semiconductor such as aluminum gallium nitride (AlGaN) or aluminum nitride (AlN) deposited on a substrate such as a sapphire substrate.
For example, AlN has a very wide band gap among those of semiconductor materials, and can efficiently take out ultraviolet light to the outside. Thus, AlN is expected as a highly efficient light emitting element substrate. However, a bulk AlN monocrystal substrate does not have sufficient crystallinity, is expensive, and is not available in a large size, so that the material of a substrate for an ultraviolet light emitting element has many problems involving crystallinity and cost.
In view of such circumstances, if a high-quality AlN thin layer can be produced on an inexpensive sapphire substrate, an ultraviolet light emitting element and a light receiving element can be produced by substantially homoepitaxially growing AlGaN using such a semiconductor substrate in which an AlN thin layer is formed on a sapphire substrate.
However, there is a large lattice mismatch between AlN and sapphire, and thus an AlN layer grown on a sapphire substrate includes many threading dislocations. Accordingly, it is difficult to obtain an AlN layer having a flat surface, which is deposited on the sapphire substrate, and also there is a problem that the AlN layer has many crystal defects. Furthermore, the crystallinity of AlGaN serving as a light emitting layer inherits the crystallinity of AlN, and thus the technology of producing AlN having low defect density is extremely important.
For example, Patent Literatures (PTLs) 1 and 2 disclose technology as methods for obtaining a high-quality layer (thin film) which has low defect density of AlN crystals. Furthermore, PTL 3 discloses technology of reducing surface roughness of substrates by performing high-temperature processing on the substrates in a state where the substrates, although they do not include AlN crystals, are placed in contact.